1. Field of the Invention
The invention relates to power management method and power management device, and more specifically to power management method and power management method for stations (STAs) in wireless LANs (local area networks).
2. Description of the Related Art
Recently, wireless local area networks (LANs) have been developed as an enhanced replacement for wired LANs. As compared to wired LANs, data-communication in a wireless LAN can be more versatile due to the flexibility of the arrangement of network stations in the area covered by the LAN, and due to the absence of cabling connections.
Wireless LANs are generally implemented according to the standard defined by the ISO/IEC 8802-11 international standard (IEEE 802.11) for wireless systems operating in the 2.4-2.5 GHz ISM (industrial, scientific and medical) band. FIG. 1 illustrates the general MAC frame format defined in IEEE 802.11. Each MAC frame includes a MAC header, a variable length frame body and a frame check sequence (FCS). As shown, the MAC header includes Frame Control, Duration/ID, Address 1, Address 2, Address 3, Sequence Control, and Address 4 fields. The address fields in MAC frame format are used to indicate the Basic Service Set identifier (BSSID), Source address (SA), Destination Address (DA), Transmitter Address (TA), and Receiver Address (RA). Thus, when receiving data frames transmitted in the wireless LANs, station (STAs) operating in a service set can detect the packets transmitted over a wireless media (WM), determining the intended recipient in accordance with the destination information thereof. Thus, a station's receiver is always powered on, waiting for data frames. However, since most STAs in the wireless network are mobile stations which may be battery powered, power management becomes an important consideration in performance analysis. IEEE 802.11 provides a mechanism to support establishment and maintenance of the power management mode of a STA, wherein a STA may be in one of two different power states, Awake and Doze. The STA in Awake state is fully powered, while the STA in Doze state is not able to transmit or receive and consumes very low power. STAs in a power-save (PS) mode provided in IEEE. 802.11 operate in the Doze state and enter the Awake state at a determined time. When a STA operates in the PS mode, the AP (access point) does not arbitrarily transmit data frames thereto, but buffers the data frames and only transmits them at designated times. The STAs that currently have buffered data frames within the AP are identified in a TIM (traffic indication map), included within all beacons generated by the AP. The STAs in the PS modes periodically monitors beacons and determine whether a data frame is buffered therefore by receiving and interpreting a TIM included therein. If the TIM element in the most recent beacon indicates directed data buffered for the STA, the STA sends PS-Poll frames to the AP. The AP transmits buffered directed data frames to a PS STA in response to the PS-Poll from that STA. FIG. 2 shows a schematic diagram illustrating the AP and STA activity in a power management operation. The top line in FIG. 1 represents the time axis, with the beacon interval shown together with a DTIM (delivery TIM) Interval of three beacon intervals. The second line depicts AP activity. The AP schedules beacons for transmission every beacon interval and transmits a TIM with every beacon. The beacons contain TIMs, some of which may be DTIM, wherein the difference between TIM and DTIM is that after a DTIM, the AP sends out the buffered broadcast/multicast packets. The third line in FIG. 1 depicts the activity of a STA operating in PS mode. The STA represented in the third lines powers up its receiver, which is indicated as a ramp-up of the receiver power and receives a TIM in the first beacon; that TIM indicates the presence of a buffered packet for the receiving STA. The receiving STA then generates a PS-Poll frame, which elicits transmission of the buffered data from the AP. Broadcast and multicast data frames are sent by the AP subsequent to the transmission of a beacon containing a DTIM.
This power management method, however, is not advantageous for STAs having data to be transmitted; since when a STA operates in PS mode, AP buffers the data frames for the STA and transmits thereto when it wakes (Awake state). Thus, for STAs having much data to be received, considerable data is buffered within the AP, which then may be lost if the data amount exceeds capability of the AP. Moreover, as shown in FIG. 1, since the first TIM indicating there is data designated to a STA, the STA enters Awake state, waiting to receive the designated data until the completion of the transmission. Thus the STA needs to operate in Awake state in the first three beacon intervals, consuming power. Moreover, in practice, STAs usually do not adopt this power management method, since the STAs need to ensure there is no data to receive or transmit when in PS mode, and it is power-consuming to receive every data frames transmitted over the WM, which may not contain the designated data.